Overall UT had the best taunts. They were simple, but to the point and very effective. The robots in UT2 were hilarious ("your programming is inferior"). I'm not sure what happened in UT3, but the quality of the voices/taunts really plummeted in that one, I think. It seems like Epic tried too hard to make them clever, funny or sophisticated.
I liked most of the Necris taunts, though ("I'll be back... again, and again", "Life is weakness" or "Let me show you hell" were really cool IMO). At the same time, they also had the taunt which made me cringe the most: "I hunger". WTF? To me, that one makes the least amount of sense of them all. Just what is the POINT of that taunt?
I wonder why Epic decided to make the Iron Gaurd females sound so god-awfully annoying. Why? Why? Do they like women with annoying voices over there? I shall never know...
Anyway, I think it would've been much better if they sounded like the merc females from UT2 (e.g. Sapphire). Those were the nicest female voices in the series to me, next to the female soldier voice from UT. I silently hope that these female voices will be replaced in the expansion. It's probably not going to happen, but it would make the game a lot easier on my ears.
Also, the game really needs another "Ha! Ha! Ha!" taunt.
Lastly, some other really cool taunts were about every single one of them used by UT's Xan. "Run, human!", "Witness my perfection", and of course "I am the alpha and the omega". But should these taunts make a return, then reserve them only for a boss character, so that alone would make him look cooler than every other character in the game. Not the way they did in UT2, where the Xan taunts were divided among the "regular" characters, making them lose a lot of their impact.